In plastic encapsulated DIP leadframes, the lead frame is designed to increase thermal dissipation with the semiconductor die mount extending across the majority of the width of the package. The width of the die pad is determined by the size of the semiconductor die, and the requirement of a die bond to the mount pad. Because of the larger die required for increased thermal dissipation, the ground bond is required to be in an area of highest shear stress. This causes fracturing of the ground bond during temperature cycling and thermal shock, and electrical failure of the device. Another problem is that the pad bond wire sags and makes contact with the die prior to molding.
Using a die pad bond with a smaller die has reduced the incident of failures, but has not totally eliminated the problem. In the case of larger die, the larger area on which to place the bond does not exist.
Another approach has been to provide holes in the die pad for locking the die pad to the mold compound, but this decreases the thermal capability that the leadframes was intended to provide, and has proven to be ineffective in locking the leadframe to the mold compound.